1. Field of the Invention
This invention relates to a novel method of bonding optical fibers to substrates and to optical assemblies thereby produced.
2. Description of the Prior Art
Optical fibers and various devices made thereof such as couplers, filters and the like must be secured to substrates for packaging purposes. Traditionally this has been done by bonding the fibers to the substrate using polymeric materials such as epoxy adhesives. Such adhesives are usually cured by exposure to heat and/or UV light, which is a precise and time consuming operation and which cannot be reversed in order to readjust or correct a defect in the packaging. Also, epoxies have very different coefficients of thermal expansion relative to the optical fibers and the substrates on which they are mounted, which presents considerable problems when is the devices are subjected to changes in temperature. Moreover, epoxies have a tendency to absorb moisture, which reduces their stability in a moist environment.
Efforts have been made to replace epoxies by other substances, such as metal oxides combined with binder components, as disclosed, for instance, in U.S. Pat. Nos. 5,500,917 and 5,682,453. This requires using intensive energy locally to heat the metal oxides to a temperature at which the binder component bums away and the residual metal-oxide based component drops sufficiently in viscosity so that it flows over and wets the surfaces of the fibers and substrates whereby such elements are bound together upon cooling. This again is a precise and time consuming as well as irreversible operation.
It has been surprisingly found that optical fibers can be efficiently bonded to a substrate with the use of a bismuth-containing alloy or solder.
Fusible alloys of bismuth-tin are already known For example, in U.S. Pat. No. 4,413,881 such alloy containing 60% tin and 40% bismuth is used to provide a hermetic seal for an optical fiber. In U.S. Pat. No. 5,061,035 a bismuth-tin fluxless solder, amongst others, is used for hermetically sealing an optical fiber array to form a sealed optical fiber bundle. Also, in U.S. Pat. No. 5,568,585 a tin/bismuth solder is mentioned amongst others as being suitable for low temperature hermetic sealing of optical fiber components.
The bismuth-tin alloys or solders have the property of slightly expanding upon solidification as disclosed, for instance, in U.S. Pat. No. 4,695,125, and thus compressing the optical fiber surrounded thereby. Also, they are essentially not subject to creep and deformation during prolonged exposure to elevated temperature or repeated temperature variations.
Such alloy or solder will not directly bond an optical fiber to a flat glass, ceramic or similar substrate since it will not form a chemical or metallurgical bond with the substrate. However, the flat substrate may be provided with metallized areas in appropriate places to which the solder will be metallurgically bonded Such metallized areas may be formed by means of e-beam evaporation, RF sputtering, chemical plating, or the like. Such metallized areas may consist, for instance, of suitable metallic pads or blanket metallization formed on the surface of the substrate layered metal pads formed, for instance, of gold-nickel-titanium layers and the like can be used. Alternatively, the whole of the substrate may be metallized since metal. layers do not require patterning in order to provide a bonding surface for the solder. Similar metallic structures are disclosed, for instance, in U.S. Pat. No. 5,061,035 already mentioned above, where they are used as a solderable metal coating at the end of each optical fiber. In the present case, they are not used to coat the fiber, but to provide a metallized surface on the substrate to which the optical fiber can be bonded.
It is also possible to bond optical fibers to the substrate without providing metallized areas as mentioned above, but by providing a cavity or recess in the substrate in which the fibers are positioned and into which the bismuth-containing alloy, such as the BiSn solder, is introduced in desired spots to bond with the fiber and with the walls of the cavity. Because, as mentioned above, bismuth-containing alloy or solder expands upon solidification, it will expand within and fill the cavity, while compressing against the optical fiber, thereby holding the fiber in place and thus bonding it to the substrate.
The bismuth-containing alloy or solder may be delivered by any suitable means, for instance, by a liquid injection device, or as a preform or may be plated on the substrate, or the like, and it may be heated to a required temperate by any suitable heat source, such as a laser, induction heating, radiant heating or the like. Also an integral source of heat can be created on the substrate by leaving a portion of the base metal exposed and by flowing electrical current therethrough.
The great advantage of the new system is that it is extremely rapid, merely requiring to place a drop of solder over the optical fiber, which solder then rapidly solidifies, and further, if adjustments are subsequently required in the optical assembly, the solder may be softened by heating and then re-solidified once the adjustments have been made.